1. Field of the Invention
The present invention relates to an apparatus configured to acquire information on a physical property or a structure of an object to be measured by using a terahertz wave, and a method of acquiring information from an object to be measured using a terahertz wave. In particular, this application relates to an apparatus for measuring a terahertz wave in the time domain (THz-TDS apparatus or THz-Time Domain Spectroscopy apparatus).
2. Description of the Related Art
A terahertz wave is typically an electromagnetic wave having a frequency component of any frequency band in a range from 0.03 THz to 30 THz. In such a frequency band, there exists a large number of distinctive absorption derived from a structure and a state of various substances such as a biomolecule. Utilizing this feature, an examination technique for performing an analysis, identification, and the like of a substance in a nondestructive manner has been developed. Such a technique is expected to be applied to a safe imaging technology, which may take place or enhance conventional X-ray technology and high-speed communication. Specifically, application terahertz technology to a tomography apparatus that visualizes inside a substance is attracting attention. In a tomography apparatus, utilizing a transmittance feature of the terahertz wave, visualization of an inner structure at a depth of several 100 μm to several 10 mm is expected.
A large number of distinctive absorption related to atmospheric moisture exits in the electromagnetic wave in the terahertz region. Therefore, in order to decrease this influence of the atmosphere, there has been often used an apparatus configuration in which a part where the terahertz wave is propagated is isolated from the atmosphere, and in which an ambient atmosphere in the part where the terahertz wave is propagated is adjusted. International patent application publication WO03/058212 discloses, in order to measure various objects to be measured without changing a state of an adjusted ambient atmosphere, an apparatus configuration in which a measuring window, through which a terahertz wave is transmitted, is provided in a part of a casing used for isolation from the atmosphere, and the object to be measured is placed in chamber on that window.
As a technique in WO03/058212, in a configuration in which an object to be measured is placed on a measuring window provided in a part of a casing, which includes a reflection measuring system, a position of the object to be measured is fixed relative to a focal position of the terahertz wave. Therefore, in this configuration, an adjustment of a relative position between the focal position of the terahertz wave and the object to be measured is limited and difficult. For example, in an optical arrangement in FIG. 10B, which is also used for describing the present invention, the following phenomenon has been confirmed as a result of a study by the present inventor. Using FIG. 10B, a case where an optical distance is measured based on a time interval between terahertz wave pulses reflected from a first interface 1018 and a second interface 1019 of the object to be measured in is studied. Here, between a case where two interfaces are within a parallel propagation region 1022 and a case where any interface exists in a light collection process region 1021, an optical distance acquired by an apparatus changes. In other words, depending on a position of the object to be measured relative to a focal position of the terahertz wave, a measuring value of optical thickness of the object to be measured may change.
Here, the parallel propagation region 1022 is a region where the terahertz wave propagates in parallel with an optical axis of the terahertz wave, and this region wave-optically corresponds to a depth of focus. Herein, the parallel propagation region 1022 is also referred to as a focal position. Likewise, the light collection process region 1021 is defined as a region of a light-collecting process of the terahertz wave.
Taking this phenomenon into account, in an apparatus configuration in which the focal position of the terahertz wave is fixed relative to the position of the object to be measured, as is the case in WO03/058212, with regard to the object to be measured placed on an apparatus casing, it will be as follows. That is, when observing a surface or an inner structure of the object to be measured, a measurement value of an optical distance may change depending on a place, and an accurate observation of the structure of the object to be measured may become difficult. As a result, measuring reliability of an apparatus for acquiring an inner structure may decrease. Herein, the optical distance, which changes according to the focal position of the terahertz wave, is referred to as a secondary propagation distance.